Method and apparatus for making and concentrating an aqueous caustic alkali

ABSTRACT

A method for concentrating an aqueous caustic alkali produced by a membrane cell process by using a single or multiple effect evaporator system in which the vapor flows in a counter direction to the aqueous caustic alkali flow and the heat recovered from the catholyte circulation line is used as part of the concentration process. In one embodiment, a catholyte heat recovery heat exchanger and flash evaporation chamber are located after the last effect of a multiple effect evaporator system. In another embodiment, the catholyte heat recovery heat exchanger and flash evaporation chamber are located prior to the single or multiple effect evaporator system. In yet another embodiment, the catholyte heat recovery process is used in conjunction with additional heat exchanger processes to further concentrate the final product as desired.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.13/650,932, filed Oct. 12, 2012, which is a Continuation of U.S. patentapplication Ser. No. 12/221,878, filed Aug. 7, 2008, now U.S. Pat. No.8,317,994, issued Nov. 27, 2012. Each patent application identifiedabove is incorporated here by reference in its entirety to providecontinuity of disclosure.

FIELD OF THE INVENTION

The present invention relates to a method of recovering heat created asa by-product of one process for use as a heat source in another process.One specific example of the present invention relates to a method ofrecovering heat from the catholyte circulation stream from a membranecell process to be used to reduce heating utility consumption in amultiple effect evaporation step.

BACKGROUND OF THE INVENTION

Electrolytic cells and cell membrane technologies have existed for manyyears. The function of electrolytic cells is to create aqueous causticalkali products, such as caustic soda (NaOH).

The method by which the electrolytic cell creates aqueous caustic alkaliproducts, specifically caustic soda, is as follows. Brine (or saltwater) is used to create caustic soda, hydrogen gas, and chlorine gas.Referring to FIG. 1, electrolytic cell 5 has anode 15 and cathode 25 andcell membrane 30 between anode 15 and cathode 25. The use of cellmembrane 30 creates anode chamber 10 and cathode chamber 20 withinelectrolytic cell 5.

Brine is fed into electrolytic cell 5 through line 50 into anode chamber10. Water is fed into electrolytic cell 5 through line 35 into cathodechamber 20. When electric current flows through electrolytic cell 5, thechlorine ions in the brine water collect around anode 15 in anodechamber 10 as chlorine gas. Sodium ions from the brine collect aroundcathode 25 and reacts with the water to form caustic soda and hydrogengas which collects in cathode chamber 20.

The chlorine gas and the depleted brine are removed from anode chamber10 through line 42 and line 40, respectively.

The aqueous caustic soda (or catholyte) and hydrogen gas is removed fromcathode chamber 20 through line 45 and line 44, respectively.

However, the concentration of the aqueous caustic alkali productscreated by the electrolytic cells is normally not high enough to meetcustomer demands or to be used efficiently in other processes.Therefore, the aqueous caustic alkali product must be concentrated to aconcentration level greater than the catholyte concentration in order tobe acceptable to sell or use in other processes. For example, manycustomers require their aqueous caustic soda (or NaOH) to have aconcentration of approximately 50% NaOH, but the concentration of NaOHcoming from the electrolytic cell is approximately 32% NaOH.

The electricity used in the electrolytic cells to create the aqueouscaustic alkali products releases heat which is absorbed by the materialswithin the cells, thereby raising its temperature. Thus, the temperatureof the catholyte from the cathode chamber of the cell and the anolytefrom the anode chamber of the cell have higher temperatures than thematerial entering the cells. Traditionally, the catholyte removed fromthe cathode chamber is split into two streams, one that is circulatedback to the cathode chamber along with water added for dilution, and onethat is to be concentrated and sold as product or used in anotherprocess within the facility. However, before the circulated catholytecan be returned to the cell, the heat added due to the electrolysis mustbe removed and the temperature of the catholyte reduced. This is mostoften done by the use of cooling water through a heat exchanger.

Further, in order to concentrate the aqueous alkali stream, heat(typically from steam) is used to cause evaporation in order to removethe excess water. Boiling point rise is a physical property of everycaustic alkali solution and increases with increased concentration anddecreases with increased vacuum. Therefore, with the higher theconcentration of caustic soda, the higher the temperature necessary inorder to cause further evaporation of the excess water from the aqueouscaustic solution.

The concentration of the aqueous caustic alkali has been done by severaldifferent methods including multiple effect evaporators, series ofevaporators, or a single evaporator. Most plants use steam as a heatingsource in a multiple effect evaporator.

U.S. Pat. No. 4,090,932 by Kazihara discloses a method of recoveringheat from the catholyte circulation line and using that as a heat sourcefor the concentration process. However, the method disclosed will notwork as described and cannot be easily modified without undueexperimentation. There are several reasons why the design disclosed byKazihara is unworkable or impracticable. Specifically, the circulatedcatholyte flow rate is excessive, the barometric condenser isincorrectly designed, and the catholyte heat exchanger is marginallydesigned.

First, the recirculated catholyte flow rate is excessive. In thedisclosed process, the examples given require that the circulatedcatholyte flow is approximately 26 times greater than the catholyte flowto be concentrated. Current cell design requires that the circulatedcatholyte is less than 8 times the catholyte to be concentrated. Thus,the circulation required by the Kazihara design is more than 3 timesthat permitted by current cell design. The high circulated catholyterate specified by the Kazihara design is impracticable since this flowis not acceptable for current cell technology.

Second, the barometric condenser is inadequately designed. Kazihara haseither specified an incorrect Boiling Point Rise or has not allowed fora sufficient minimum temperature driving force for heat transfer for thebarometric condenser. Boiling Point Rise is a physical property of anyboiling liquid. Under atmospheric pressure water boils at 212° F. andthe boiling temperature for 50% NaOH is 290° F., therefore, the boilingpoint rise of 50% NaOH is 78° F. (290° F.−212° F.). Furthermore, thewater vapor evaporated under atmospheric pressure from the 50% NaOH canbe condensed at 212° F. and this temperature is defined as the saturatedvapor temperature (also referred to as dew point). Under high vacuum,where water boils at 95° F., the boiling temperature for 50% NaOH is165° F. and the boiling point rise of 50% NaOH is 70° F. (165° F.−95°F.). The water vapor evaporated under this high vacuum from the 50% NaOHcan be condensed at 95° F. (the saturated vapor temperature).

Kazihara specifies a boiling temperature of 74° C. (165° F.) for 50%NaOH in U.S. Pat. No. 4,090,932 FIG. 6 with the cooling water enteringthe barometric condenser at 30° C. (86° F.) and exiting at 34° C. (93°F.). In order to condense water vapor in a barometric condenser, thetemperature difference between the exiting cooling water and thesaturated vapor temperature must be at least 6° F. This indicates thatKazihara chose a Boiling Point Rise for 50% NaOH of 66° F. (165° F.−93°F.−6° F.=66° F. (37° C.)) rather than 70° F. which is supported bypublished data or that Kazihara selected a temperature differencebetween the exiting cooling water and the saturated vapor temperature of2° F. that would use the correct Boiling Point Rise of 70° F. (165°F.−93° F.−2° F.=70° F. (37° C.)). Either the Boiling Point Rise has beenincorrectly calculated or an inadequate temperature driving force hasbeen specified. Regardless, the barometric condenser can not be designedas specified.

Third, the catholyte heat exchanger is marginally designed. Even if 50%NaOH is maintained at 165° F. (74° C.), the design disclosed by Kaziharais unworkable in practice. In FIG. 6 Kazihara specifies a catholytetemperature of 90° C. (194° F.) and a temperature to the evaporator(from the catholyte heat exchanger (6)) of 86° C. (187° F.). This meansthe temperature difference (ΔT) on that end of the exchanger is 194°F.−187° F.=7° F. The given circulation rates indicate that indeedKazihara expects to maintain the 7° F. ΔT on both ends of the heatexchanger. Industry standard is to maintain ΔT of at least 10° F. oneach end of the catholyte heat exchanger. A catholyte heat exchangerdesigned with a ΔT smaller than 10° F. tends to be excessively large andalso difficult to operate.

In summary, this is an unworkable design which suffers from three majorproblems. Furthermore, any changes to fix one of these problems makesthe other problems worse. All embodiments of the invention disclosed inKazihara suffer from similar problems because in all cases they aretrying to transfer the heat from the circulated catholyte to 50% NaOH.

U.S. Pat. No. 4,105,515 by Ogawa discloses a process for electrolysis ofalkali halide in which the electrolytic cell is maintained at higherthan atmospheric pressures and includes a multi stage double effectevaporator to concentrate the catholyte. First, current state of the artelectrolytic cells do not operate above atmospheric pressure. Second,the disclosure only allows for the NaOH to be concentrated to 43% NaOHwhich is not economical to be sold as a product due to transportationand handling considerations. In most instances a higher concentration,of approximately 50% NaOH, is required. Third, the procedure disclosedwill not function properly or give the desired results if the cellpressure is not higher than atmospheric pressure.

The present invention allows for decreasing the amount of steam toconcentrate the catholyte generated from the electrolytic cell whilerecovering the heat generated by the electrolytic cell and maintaining acirculation rate of approximately 8 times the rate being concentrated.The first embodiment of this invention further allows for increasedproduction when using prior equipment.

SUMMARY OF THE INVENTION

Sodium hydroxide (NaOH), also known as caustic soda, is used in manyindustries, some of which include the manufacture of pulp and paper,textiles, drinking water, soaps, and detergents. It is also the mostused base in chemical laboratories. Caustic soda can either be sold asan aqueous solution or as a solid. Due to transportation and handlingconsiderations, most caustic soda is sold as an aqueous solution whichis at least 50% NaOH.

One method of creating an aqueous solution is through the use of amembrane cell process in which a cation membrane is between a cathodeand anode within the cell. Brine is fed into the cell in the anodechamber and water is fed into the cathode chamber. When electricity goesthrough the cell, the sodium chloride (NaCl) within the brine isseparated and the chloride ions collect around the anode and the sodiumions pass through the cation membrane and collect around the cathode.After passing through the membrane, the sodium ions react with the waterto form hydrogen gas and caustic soda. The chloride ions at the anodeform chlorine gas.

Current membrane cell technology has determined that effective celloperation results in catholyte of aqueous caustic soda of approximately32% NaOH and an approximate temperature of 190° F. The exit temperatureof the catholyte is higher than the entrance temperature of the brineand water because of the absorption of heat generated by the electricitythrough the cathode and anode. Further, effective membrane celloperation provides for a circulation line in which a portion of thecatholyte removed from the cell is returned to the cell. Thiscirculation rate should be less than 8 times the rate removed forconcentration. However, prior to reintroduction to the cell, thecirculated catholyte must be cooled to remove the added heat.

While part of the catholyte is being circulated back to the cell, therest will be concentrated by removing the excess water. Theconcentration process is accomplished through a single or multipleeffect evaporator system and a catholyte heat recovery evaporator step.Current evaporator technology has determined that a maximum of threeeffects can be used to concentrate catholyte to approximately 50% NaOH.The catholyte heat recovery evaporator step includes one or more heatexchangers, one or more flash chambers, and/or one or more evaporatorbodies along with a surface condenser or barometric condenser.

In one embodiment, the catholyte to be concentrated flows through thethird effect of the triple effect evaporator, through the catholyte heatrecovery evaporator step, through the second effect evaporator, and thenthrough the first effect evaporator. Through each effect and thecatholyte heat recovery evaporator, water is evaporated and theconcentration of NaOH is increased. During each effect and the catholyteheat recovery evaporator step, the catholyte is heated to evaporate thewater.

In the first effect, the heat source is condensing steam. In the secondeffect, the heat source is the condensing water vapor released from thefirst effect. In the third effect, the heat source is the condensingwater vapor released from the second effect. In the catholyte heatrecovery evaporator step, the heat source is the circulating catholyte.Therefore, the released vapors and the aqueous caustic soda are flowingin opposite directions to each other.

Further, the 50% NaOH from the first effect is used as a heat source forpreheater heat exchangers of the aqueous caustic soda going into thefirst effect and going into the second effect. The steam condensed fromthe first effect is used as a heat source for preheater heat exchangersfor the aqueous caustic soda going into the first effect and going intothe second effect. The water vapor that had been condensed from thesecond effect is used as a heat source for a preheater heat exchangerfor the aqueous caustic soda going into the second effect.

By recovering heat from the circulation line of the catholyte back tothe cell, and the condensate lines from the effects, heating costs forthe entire process are reduced. Further, by adding the catholyte heatrecovery evaporation to an existing evaporator system, the amount ofevaporation needed from each effect to reach the desired concentrationis reduced, thereby increasing the potential to increase production whencompared to the system without the catholyte heat recovery evaporatorstep.

In another embodiment, the catholyte heat recovery evaporator step isprior to the third effect. The catholyte to be concentrated enters aflash chamber which reduces the boiling temperature as the pressure isreduced allowing vapor to be removed and is further heated by acirculation line for the flash chamber that is heated by the catholytecirculating back to the membrane cell. With this configuration, thepotential to increase production in an existing evaporator system ismore difficult because additional heat transfer area must be addedbecause of the increase in boiling point rise in all effects. Thisembodiment would be most beneficial if retrofitting an existing plantwhere the evaporation portion of the process is not located close to thecells.

In yet another embodiment, the catholyte heat recovery evaporator stepis used as the initial step in conjunction with other heat exchangerprocesses well known in the art to further concentrate the final productto the desired concentration to produce concentrations greater than 50%,including both 70% and 100% commercial grades of caustic soda.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow sheet for a typical electrolytic cell as used in theinvention.

FIG. 2A is a flow sheet for one embodiment of this invention.

FIG. 2B is a flow sheet for one embodiment of this invention.

FIG. 3A is a mass balance for the embodiment of the invention shown inFIG. 2.

FIG. 3B is a mass balance for the embodiment of the invention shown inFIG. 2.

FIG. 3C is a mass balance for the embodiment of the invention shown inFIG. 2.

FIG. 3D is a mass balance for the embodiment of the invention shown inFIG. 2.

FIG. 4A is a flow sheet for another embodiment of this invention.

FIG. 4B is a flow sheet for another embodiment of this invention.

FIG. 5A is a mass balance for the embodiment of the invention shown inFIG. 4.

FIG. 5B is a mass balance for the embodiment of the invention shown inFIG. 4.

FIG. 5C is a mass balance for the embodiment of the invention shown inFIG. 4.

FIG. 5D is a mass balance for the embodiment of the invention shown inFIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

One way that aqueous alkali and aqueous caustic alkali are produced isthrough the use of cell membranes. The preferred feed materials toelectrolytic cells include KCl (potassium chloride) producing KOH (alsoknown as caustic potash) and NaCl (sodium chloride) producing causticsoda. Since NaCl is the most preferred feed material, caustic soda willbe used to describe the method of use for the invention; however, otheralkalis and caustic alkalis may be substituted. Other alkali metalsinclude: lithium, rubidium, cesium, and francium.

The creation of caustic soda through use of cell membranes is known inthe art as discussed in relation to FIG. 1. Due to the electric currentpassing through electrolytic cell 5, the temperature of the contents inanode chamber 10 and cathode chamber 20 are elevated.

The catholyte removed from cathode chamber 20 is split into aconcentration stream and circulation stream. The concentration streamflows through line L1. The circulation stream flows through line CATH1and is eventually fed back into cathode chamber 20 through line CATH2.The circulation stream flow rate through line CATH1 should equal to orless than eight times the concentration stream flow rate through lineL1. During the circulation process, heat absorbed by the catholyte whilein electrolytic cell 5 is released.

FIGS. 2A and 2B describe one embodiment of the invention by which theheat from the circulation stream is recovered and the aqueous causticsoda in the concentration stream is concentrated. FIGS. 3A-3D are atable which illustrates an example of a mass balance for the embodimentillustrated in FIGS. 2A and 2B for the production of 1,000 ECU(Electrochemical Units) (which amounts to approximately 1,000 short tonsper day of chlorine and approximately 1,120 short tons per day ofcaustic soda). In this embodiment, the calculated values have beenrounded off to approximate values.

Referring to FIGS. 2A and 2B, the catholyte removed from cathode chamber20 in FIG. 1 is split and flows through line L1A and line CATH1A. Thecatholyte in line L1A will be concentrated while the catholyte in lineCATH1A is circulated back to cathode chamber 20. Approximately 7 timesthe catholyte in line L1A is circulated through line CATH1A.

In this embodiment, the catholyte to be concentrated will go through athree effect evaporation system and a catholyte heat recovery evaporatorstep; however, this invention could be practiced with either a doubleeffect or single effect evaporation system.

Line L1A is connected to third effect vapor body 485A, which isconnected to third effect heating element 480A. The aqueous caustic sodaflowing in line L1A has a temperature of approximately 190° F., a flowrate of approximately 291,700 lbs/hr and a concentration ofapproximately 32% caustic soda by weight.

Third effect heating element 480A and third effect vapor body 485A canbe constructed of stainless steel.

The heat source for third effect heating element 480A is the vaporproduced from second effect vapor body 495A which flows through vaporline V2A and vapor from process condensate receiver 90A which flowsthrough vapor line V5A. Vapor lines V5A and V2A combine to form line V6Awhich is connected to third effect heating element 480A. The vapor invapor line V2A has an approximate temperature of 220° F., an approximatepressure of 3.9 psia and an approximate flow rate of 21,800 lbs/hr. Thevapor flowing through vapor line V5A is small in comparison to thevapors in vapor line V2A and is primarily intended to recover heat whichwas not recovered in the second effect condensate preheater 530A. Theflow rate of the vapor in vapor line V5A is approximately 300 lbs/hrwith a temperature of approximately 157° F. and an approximate pressureof 3.9 psia. Therefore, the approximate temperature, and flow rate ofthe vapor in vapor line V6A is approximately 22,100 lbs/hr with anapproximate temperature of 219° F. and an approximate pressure of 3.9psia.

The vapor from vapor line V6A, after being condensed, exits third effectheating element 480A through condensate line C6A, which is alsoconnected to process condensate receiver 90A. The condensate incondensate line C6A has an approximate temperature of 155° F. and anapproximate flow rate of 22,100 lbs/hr.

The vapor released in third effect vapor body 485A due to concentrationof the aqueous caustic soda exits through vapor line V3A. The vapor invapor line V3A has an approximate temperature of 145° F., an approximatepressure of 1.13 psia and an approximate flow rate of 30,100 lbs/hr.

The concentrated aqueous caustic soda exits third effect heating element480A and third effect vapor body 485A through line L20A at anapproximate flow rate of 2,368,900 lbs/hr, an approximate temperature of148° F., and an approximate concentration of 35.7% caustic soda byweight.

Line L20A splits into two lines L2A and L21A. Line L21A is a circulationline for third effect heating element 480A. The aqueous caustic soda inline L21A is pumped through pump 10A at an approximate flow rate of2,107,300 lbs/hr and re-enters third effect heating element 480Acreating a circulation stream. The concentrated aqueous caustic soda inline L2A has an approximate temperature of 148° F., an approximate flowrate of 261,600 lbs/hr, and an approximate concentration of 35.7%caustic soda by weight.

Line L2A joins with line L22A to create line L23A. The aqueous causticsoda in line L2A is pumped through pump 20A.

The aqueous caustic soda in line L23A enters flash chamber 520A with anapproximate concentration of 40% caustic soda, an approximate flow rateof 2,166,000 lbs/hr, and an approximate temperature of 176° F. Theaqueous caustic soda that enters flash chamber 520A is concentrated asvapor is released. The released vapor leaves flash chamber 520A throughvapor line V4A. The concentrated caustic soda leaves flash chamber 520Athrough line L24A at an approximate concentration of 40.6% caustic soda,an approximate flow rate of 2,134,200 lbs/hr, and an approximatetemperature of 157° F. The vapor in vapor line V4A has an approximatetemperature of 157° F., an approximate pressure of 1.04 psia and anapproximate flow rate of 31,800 lbs/hr.

The aqueous caustic soda in line L24A is pumped through pump 40A priorto splitting into line L31A and line L13A.

Line L31A is connected to catholyte heat exchanger 510A. The aqueouscaustic soda in line L31A enters catholyte heat exchanger 510A at anapproximate flow rate of 1,904,500 lbs/hr and an approximate temperatureof 157° F. and exits through line L22A with an elevated temperature ofapproximately 180° F. Line L22A joins with line L2A to form line L23Awhich is connected to flash chamber 520A.

The heat source for catholyte heat exchanger 510A is the catholyte beingcirculated hack to cathode chamber 20, shown in FIG. 1, at anapproximate flow rate of 2,900 gal/min (or approximately 1,896,700lbs/hr). The catholyte enters catholyte heat exchanger 510A through lineCATH1A with an approximate temperature of 190° F. and exits through lineCATH2A with an approximate temperature of 167° F. The catholyte iscooled as it progresses through catholyte heat exchanger 520A. LineCATH2A transports the cooled catholyte back to electrolytic cell 5 andcathode chamber 20.

As stated previously, line L24A splits into lines L31A and L13A. Theaqueous caustic soda to be further concentrated flows through line L13A.The aqueous caustic soda in line L13A has an approximate temperature of157° F., an approximate flow rate of 229,800 lb/hr, and an approximateconcentration of 40.6% NaOH. Line L13A splits into lines L25A and L26A.Line L25A is connected to second effect condensate preheater 530A andthe flow rate of the aqueous caustic soda is approximately 50,000lbs/hr. The concentrated aqueous caustic soda flowing through line L25Ais heated in second effect condensate preheater 530A and exits throughline L3A.

The aqueous caustic soda in Line L3A has an approximate flow rate of50,000 lbs/hr, an approximate temperature of 194° F. and an approximateconcentration of 40.6% NaOH. Line L3A is connected to second steamcondensate preheater 540A. Concentrated aqueous caustic soda which goesinto second steam condensate preheater 540A is heated and exits throughline L4A. The temperature of the aqueous caustic soda in line L4A isapproximately 226° F.

The heat source for second effect condensate preheater 530A is thecondensed vapor from second effect heating element 490A which exitssecond effect heating element 490A through condensate line C4A. Thecondensed vapor enters second effect condensate preheater 530A throughcondensate line C4A and exits through condensate line C5A. Condensateline C5A connects to process condensate receiver 90A. The condensedvapor flowing through condensate lines C4A and C5A have an approximateflow rate of 21,300 lbs/hr of water. The approximate temperature of thecondensed vapor in condensate line C4A is 237° F. which is cooled toapproximately 167° F. when it reaches condensate line C5A.

The heat source for second steam condensate preheater 540A is thecondensed steam from first effect heating element 500A after it hasproceeded through first steam condensate preheater 560A. The condensedsteam enters second steam condensate preheater 540A through condensateline C2A and exits second steam condensate preheater 540A throughcondensate line C3A. The condensed steam enters second steam condensatepreheater 540A with an approximate temperature of 250° F. and anapproximate flow rate of 35,200 lbs/hr and exits at an approximatetemperature of 210° F.

The concentrated aqueous caustic soda in line L26A flows through secondproduct preheater 550A at a flow rate of approximately 179,800 lbs/hr.The temperature of the concentrated aqueous caustic soda in line L26A iselevated and exits second product preheater 550A through line L5A. Thetemperature of the aqueous caustic soda in line L5A is approximately229° F. and the flow rate of the aqueous caustic soda is approximately179,800 lbs/hr.

The heat source for second product preheater 550A is the final productaqueous caustic soda. The final product aqueous caustic soda has anelevated temperature due to the concentration process. By using secondproduct preheater 550A, the heat used to create the final productaqueous caustic soda is recovered to assist in the concentration ofother aqueous caustic soda. The final product aqueous caustic sodaenters second product preheater 550A through line L10A and exits throughline L11A. The approximate flow rate of the final product aqueouscaustic soda (50% NaOH) is approximately 186,700 lbs/hr through linesL10A and L11A. The temperature of the final product aqueous caustic sodadecreases from approximately 260° F. in line L10A to approximately 180°F. in line L11A.

Line L5A and L4A are joined into line L30A. Line L30A containsconcentrated aqueous caustic soda which will be concentrated further.The aqueous caustic soda in line L30A has an approximate temperature of228° F., an approximate flow rate of 229,800 lbs/hr, and an approximateconcentration of 40.6% caustic soda by weight. Line L30A joins with lineL18A, which is a circulation line for second effect heating element490A, to form line L19A. The aqueous caustic soda in Line L18A has anapproximate flow rate of 229,500 lbs/hr, an approximate temperature of215° F., and an approximate concentration of 44.9% caustic soda byweight. When line L18A joins with line L30A, the aqueous caustic sodaflowing through line L19A has an approximate flow rate of 459,200lbs/hr, an approximate temperature of 221° F., and an approximateconcentration of 42.7% caustic soda by weight.

Line L19A is connected to second effect heating element 490A. Secondeffect heating element 490A is connected to second effect vapor body495A. In second effect heating element 490A, the aqueous caustic soda isheated to increase its temperature and vaporize water contained therein.The water which is vaporized exits second effect vapor body 495A,through vapor line V2A. As a result of the vaporization, the aqueouscaustic soda becomes more concentrated.

The heat source for second effect heating element 490A is the vaporcreated from the concentration of the aqueous caustic soda and releasedfrom first effect vapor body 505A. The vapor enters second effectheating element 490A through vapor line V1A and, after being condensed,exits through condensate line C4A. As stated previously, the condensedvapor from second effect heating element 490A exits through condensateline C4A and flows through second effect condensate preheater 530A inorder to heat a portion of the caustic flowing to second effect heatingelement 490A. The temperature of the water vapor in vapor line V1A isapproximately 320° F. at an approximate pressure of 24.1 psia and thevapor has an approximate flow rate of 21,300 lbs/hr.

The vapor released from the concentration process exits second effectvapor body 495A through vapor line V2A. The vapor is used as the heatsource for third effect heating element 480A and enters third effectheating element 480A through vapor line V6A.

The concentrated aqueous caustic soda leaves second effect heatingelement 490A and second effect vapor body 495A through line L17A with anapproximate flow rate of 437,400 lbs/hr, an approximate temperature of215° F., and an approximate concentration of 44.9% caustic soda byweight.

Line L17A splits into line L6A and line L18A. Line L18A connects withline L30A to circulate back to second effect heating element 490A. Theaqueous caustic soda in line L18A is pumped through pump 70A.

Line L6A contains aqueous caustic soda that is being furtherconcentrated through the first effect. The aqueous caustic soda in lineL6A has a flow rate of approximately 208,000 lbs/hr and a temperature ofapproximately 215° F. The approximate concentration of caustic soda inthe aqueous caustic soda in line L6A is 44.9%. Line L6A splits into twolines, line L27A and line L28A, and the aqueous caustic soda flowingthrough these lines is split into approximately 65,000 lbs/hr in lineL27A and approximately 143,000 lbs/hr in line L28A. The aqueous causticsoda flowing in line L6A is pumped by pump 60A.

Line L27A is connected to first steam condensate preheater 560A. Theconcentrated aqueous caustic soda flowing through line L27A enters firststeam condensate preheater 560A and is heated. The concentrated aqueouscaustic soda exits first steam condensate preheater 560A through lineL7A at a flow rate of approximately 65,000 lbs/hr and a temperature ofapproximately 287° F.

The heat source for first steam condensate preheater 560A is thecondensed steam from first effect heating element 500A. The condensedsteam enters through condensate line C1A and exits through condensateline C2A. The flow rate of the condensed steam is approximately 35,200lbs/hr and is reduced from approximately 362° F. to approximately 250°F. as it flows through first steam condensate preheater 560A. While thecondensed steam is cooled as it proceeds through first steam condensatepreheater 560A, the concentrated aqueous caustic soda is heated.

Line L28A connects to first product preheater 570A. The concentratedaqueous caustic soda enters first product preheater 570A through lineL28A, is heated, and exits through line L8A with an approximatetemperature of 285° F. The heat source for first product preheater 570Ais the final product aqueous caustic soda from first effect heatingelement 500A and first effect vapor body 505A which has completed theconcentration process. The final product aqueous caustic soda entersfirst product preheater 570A through line L9A, is cooled, and exitsthrough line L10A. The final product aqueous caustic soda is pumped intofirst product preheater by pump 50A.

The concentrated aqueous caustic soda flowing through first productpreheater 570A has a flow rate of approximately 143,000 lbs/hr andenters at approximately 215° F. and exits at approximately 285° F. Thefinal product aqueous caustic soda enters first product preheater 570Aat an approximate temperature of 320° F. and exits at an approximatetemperature of 260° F. The flow rate of the final product caustic sodais approximately 186,700 lbs/hr.

Line L7A and line L8A are joined to create line L29A. Line L29A joinswith line L15A to form line L16A. Line L16A connects to first effectheating element 500A. The aqueous caustic soda in line L29A has anapproximate flow rate of 208,000 lbs/hr and approximate temperature of286° F. When the aqueous caustic soda from line L29A joins with theaqueous caustic soda from line L15A, which has an approximate flow rateof 119,200 lbs/hr, an approximate temperature of 320° F., and anapproximate concentration of 50% caustic soda, the resulting aqueouscaustic soda has an approximate flow rate of 327,200 lbs/hr, anapproximate temperature of 298° F., and an approximate concentration of46.7% caustic soda and is flowing through line L16A.

First effect heating element 500A is connected to first effect vaporbody 505A. Water vapor is released from the concentrated aqueous causticsoda thereby further concentrating the aqueous caustic soda into thefinal concentration of caustic soda to be sold or used in otherprocesses. The final product aqueous caustic soda exits first effectheating element 500A and first effect vapor body 505A through line L14A.The aqueous caustic soda flowing through line L14A has an approximateconcentration of 50% caustic soda, an approximate flow rate of 305,900lbs/hr, and an approximate temperature of 320° F.

The water vapor released during the concentration of the caustic sodaexits first effect vapor body 505A through vapor line V1A. The vapor isused as the heat source for the second effect heating element 490A.

The heat source for first effect heating element 500A is steam andenters first effect heating element 500A through steam line S1A at anapproximate flow rate of 35,200 lbs/hr, a pressure of approximately194.7 psia and temperature of approximately 380° F. The condensed steamexits first effect heating element 500A through condensate line C1A. Aspreviously discussed, the condensed steam is further used to heat thefeeds going into first effect heating element 500A and second effectheating element 490A.

Line L14A is split into line L15A and line L9A. Line L15A is a portionof a circulation line for first effect heating element 500A. It isconnected to line L29A to form line L16A which is connected to firsteffect heating element 500A. The aqueous caustic soda which flowsthrough line L15A is pumped by pump 80A.

The final product aqueous caustic soda is cooled through first productpreheater 570A and second product preheater 550A. However, it shouldstill be cooled further. Therefore, the final product aqueous causticsoda flows through product cooler 100A. Line L11A which is connected toexit of second product preheater 550A is also connected to productcooler 100A. After the final product aqueous caustic soda flows throughproduct cooler 100A, it exits through line L12A and has a temperature ofapproximately 120° F. The cooling source for product cooler 100A iscooling water. The cooling water enters through cooling water line CW5Aat an approximate flow rate of 1,000 gal/min and an approximatetemperature of 85° F. and exits through cooling water line CW6A with anapproximate temperature of 102° F.

The vapor released from third effect vapor body 485A travels throughvapor line V3A into surface condenser 305A where the vapor is cooled andcondensed. A barometric condenser could also be used for this service.The cooling source for surface condenser 305A is cooling water whichenters surface condenser 305A through cooling water line CW1A with anapproximate flow rate of 6,000 gal/min and an approximate temperature of85° F. The cooled and condensed vapor exits surface condenser 305Athrough condensate line C7A at an approximate flow rate of 30,100 lbs/hrand an approximate temperature of 105° F. Condensate line C7A isconnected to process condensate receiver 90A.

The cooling water that has been heated as it proceeded through surfacecondenser 305A exits through cooling water line CW4A with an approximatetemperature of 96° F. The vapor released from flash chamber 520A exitsflash chamber 520A through vapor line V4A. Vapor line V4A connects tosecond surface condenser 315A. A barometric condenser could also be usedfor this service. Vapor enters second surface condenser 315A and iscondensed by cooling water that enters second surface condenser 315Athrough cooling water line CW2A at an approximate flow rate of 7,000gal/mm and an approximate temperature of 85° F.

The cooling water is heated as it proceeds through second surfacecondenser 315A exits through cooling water line CW3A with an approximatetemperature of 95° F.

The condensed vapor exiting second surface condenser 315A exits throughcondensate line C9A. Condensate line C9A is connected to processcondensate receiver 90A. Condensate flowing through condensate line C9Ahas an approximate flow rate of 31.800 lbs/hr and an approximatetemperature of 105° F.

Process condensate receiver 90A collects condensate from the differenteffects and stages of the evaporation process. Any remaining vaporcontained in condensate lines C5A, C9A, C7A, or C6A is released fromprocess condensate receiver through vapor line V5A and is used as a heatsource for third effect heating element 480A.

The condensed vapor, or water, is pumped out of process condensatereceiver 90A by pump 30A through condensate line C8A at an approximateflow rate of 105,000 lbs/hr and an approximate temperature of 126° F.The water can be used many different ways within the facility, includingas the water source for the electrolytic cell.

Cooling water flowing through cooling water lines CW3A, CW4A and CW6A isreturned to the cooling tower to be cooled and reused in theconcentration process or other cooling required at other locationswithin the facility.

First effect heating element 500A, first effect vapor body 505A, secondeffect heating element 490A, second effect vapor body 495A, secondeffect condensate preheater 530A, second product preheater 550A, secondsteam condensate preheater 540A, first steam condensate preheater 560A,and first product preheater 570A, and all lines in which aqueous causticsoda above approximately 40% NaOH flows should be constructed of amaterial resistant to corrosion by caustic soda, such as nickel. Theflash chamber 520A, catholyte heat exchanger 510A, could be constructedof a higher nickel stainless steel or nickel.

The above example gives one embodiment which attempts to optimize theuse and recovery of heat from different heat sources as part of theconcentration process. However, those skilled in the art will recognizethat this invention can be practiced without the use of second effectcondensate preheater 530A, second steam condensate preheater 540A,second product preheater 550A, first steam condensate preheater 560A,first product preheater 570A, and product cooler 100A, or canincorporate any combination thereof.

Those skilled in the art will recognize that catholyte heat exchanger510A can consist of two or more heat exchangers arranged in eitherseries or parallel, and flash chamber 520A can consist of two or moreflash chambers connected to two or more vapor bodies arranged in eitherseries or parallel.

Further, those skilled in the art will recognize that the concentrationsof caustic soda flowing through this one embodiment can vary in actualpractice. For example, the concentration of the aqueous caustic sodaflowing through line L1A can range from approximately 31.0% toapproximately 33.0% caustic soda by weight. The concentration of theaqueous caustic soda flowing from third effect heating element 480A andthird effect vapor body 485A and through line L2A can range fromapproximately 34.6% to approximately 36.8% caustic soda by weight. Theconcentration of the aqueous caustic soda flowing from flash chamber520A and through line L13A can range from approximately 39.4% toapproximately 41.8% caustic soda by weight. The concentration of theaqueous caustic soda flowing from second effect heating element 490A andsecond effect vapor body 495A and through line L6A can range fromapproximately 43.5% to approximately 46.3% caustic soda by weight. Theconcentration of the final product aqueous caustic soda flowing fromfirst effect heating element 500A and first effect vapor body 505A andthrough line L9A can range from approximately 48.5% to approximately51.5% caustic soda by weight.

Although caustic soda is most commonly sold as a 50% concentrationproduct, this embodiment of the invention may be employed as an initialstep toward achieving caustic soda concentrations greater than 50%,including the 70% and 100% commercial grades of caustic soda.

FIGS. 4A and 4B are a flow diagram for another embodiment of theinvention. FIGS. 5A-5D are a table showing mass balance and temperaturesof an example of this embodiment for the production of 1,000 ECU a day.This embodiment would be the preferred embodiment when the electrolyticcells as discussed in FIG. 1 are not located in close proximity to theconcentration process equipment. In this embodiment, approximately 5times the catholyte concentration flow is being circulated back to theelectrolytic cell.

Referring to FIGS. 4A and 4B, the catholyte removed from cathode chamber20 in FIG. 1 which is to be concentrated flows through line L1B at anapproximate rate of 291,700 lbs/hr, an approximate temperature of 190°F., and an approximate concentration of 32% caustic soda by weight. LineL1B is attached to flash chamber 520B. Catholyte flows through line L1Binto flash chamber 520B and water vapor escapes from the catholyte dueto the decrease in pressure causing the temperature of remaining aqueouscaustic soda to be decreased and leaving a higher concentration ofaqueous caustic soda.

The concentrated aqueous caustic soda exits flash chamber 520B throughline L23B and is pumped by pump 40B through line L23B. The aqueouscaustic soda leaving flash chamber 520B has an approximate flow rate of1,369,600 lbs/hr, an approximate concentration of 37.1% caustic soda,and an approximate temperature of 148° F. Line L23B splits into twolines, line L24B and line L13B.

Line L24B is connected to catholyte heat exchanger 510B. The aqueouscaustic soda flows through catholyte heat exchanger 510B at a flow rateof approximately 1,369,000 lbs/hr and is heated to approximately 180° F.It exits catholyte heat exchanger 510B through line L22B. Line L22B isconnected to flash chamber 520B and allows the heated aqueous causticsoda to flow into flash chamber 510B. Line L22B and line L23B form acirculation loop for flash chamber 520B.

The heat source for catholyte heat exchanger 510B is the circulationline from cathode chamber 20 in FIG. 1. Line CATH1B is attached tocatholyte heat exchanger 510B and the catholyte from the electrolyticcells flows through line CATH1B into catholyte heat exchanger 510B. Thecatholyte s cooled and exits through line CATH2B. Line CATH2B brings thecooled catholyte back to the electrolytic cell. The catholyte flowsthrough lines CATH1B and CATH2B at an approximate rate of 2,200 gal/mm.The catholyte in line CATH1B has an approximate temperature of 190° F.and is cooled to approximately 160° when it reaches line CATH2B. Theflow rate of catholyte in CATH2B is approximately 5 times the flow rateof catholyte in line L1B.

The water vapor released in flash chamber 520B exits flash chamber 520Bthrough vapor line V4B. Vapor line V4B is connected to second surfacecondenser 315B and the vapor flowing through vapor line V4B is condensedback to water in second surface condenser 315B. A barometric condensercould also be used for this service. The water vapor in vapor line V4Bhas an approximate flow rate of 40,000 lbs/hr at an approximatetemperature of 147° F. and an approximate pressure of 1.17 psia.

Flash chamber 520B and catholyte heat exchanger 510B can be constructedof stainless steel due to the concentration of caustic soda in contactwith these pieces of equipment.

Line L13B is connected to third effect vapor body 485A and the aqueouscaustic soda which is to be further concentrated flows through line L13Binto third effect vapor body 485B at an approximate flow rate of 251,600lbs/hr and an approximate temperature of 148° F. The approximateconcentration of caustic soda in the aqueous caustic soda in line L13Bis 37.1%.

Third effect vapor body 485B is connected to third effect heatingelement 480B. The concentrated aqueous caustic soda that flows throughline L13B is heated in third effect heating element 480B causing watervapor to be released and leaving a further concentrated aqueous causticsoda.

The vapor released from third effect vapor body 485B exits through vaporline V3B, at an approximate rate of 18,400 lbs/hr, an approximatepressure of 1.17 psia and an approximate temperature of 157° F. Vaporline V3B is connected to surface condenser 305B and the vapor flowingthrough vapor line V3B is condensed back to water in surface condenser305B. A barometric condenser could also be used for this service.

The concentrated aqueous caustic soda exits third effect vapor body 485Band exits from third effect heating element 480B through line L20B at anapproximate flow rate of 2,402,600 lbs/hr, an approximate temperature of157° F., and an approximate concentration of 40% caustic soda by weight.

Line L20B splits into lines L2B and L21B. Line L21B is a circulationloop for third effect heating element 480B. The aqueous caustic soda inline L21B is pumped to the third effect heating element by pump 10B atan approximate flow rate of 2,169,300 lbs/hr.

The heating source for third effect heating element 480B is the vaporreleased from second effect heating element 495B flowing through vaporline V2B and any vapor released from process condensate receiver 90Bflowing through vapor line V5B. Vapor lines V5B and V2B join to formvapor line V6B which is connected to third effect heating element 480B.The vapor flow through vapor line V2B is approximately 22,000 lbs/hr atan approximate temperature of 226° F. and an approximate pressure of 5.7psia. The vapor from process condensate receiver 90B through vapor lineV5B is mainly a line to bleed any vapors released into processcondensate receiver 90B from the collection of the different condensatelines. The addition of any vapor through vapor line V5B is minimal andthus, the composition of vapor flowing through vapor line V6B issubstantially the same as the vapor flowing through vapor line V2B.

The condensed vapor that exits third effect heating element 480B flowsthrough condensate line C6B. Condensate line C6B connects to processcondensate receiver 90B. The condensed vapor in condensate line C6B hasa temperature of approximately 167° F. and a flow rate of approximately22,000 lbs/hr.

Line L2B splits into lines L25B and L26B. The caustic flowing throughline L2B is pumped into line L25B and line L26B by pump 20B. Theconcentrated aqueous caustic soda flowing in line L2B has an approximatetemperature of 157° F., an approximate flow rate of 233,300 lbs/hr, andan approximate concentration of 40% caustic soda by weight. The aqueouscaustic soda in line L2B is split approximately 50,000 lbs/hr into lineL25B and approximately 183,300 lbs/hr into line L26B.

Line L25B connects to second effect condensate preheater 530B. Aqueouscaustic soda flowing through line L25B is heated as it flows throughsecond effect condensate preheater 530B and exits through line L3B. Thetemperature of the aqueous caustic soda leaving second effect condensatepreheater 530B is approximately 197° F. and has a flow rate ofapproximately 50,000 lbs/hr.

Second effect condensate preheater 530B uses the condensed vapor fromsecond effect heating element 490B as the heat source. The condensedvapor flows from second effect heating element 490B through condensateline C4B into second effect condensate preheater 530B. The condensedvapor is cooled further as it proceeds through second effect condensatepreheater 530B and exits through condensate line C5B into processcondensate receiver 90B. The condensed vapor enters second effectcondensate preheater 530B with a temperature of approximately 237° F.and exits at a temperate of approximately 158° F. The approximate flowrate of the condensed vapor through second effect condensate preheater530B is 24,700 lbs/hr.

Line L3B is also connected to second steam condensate preheater 540B.The heated aqueous caustic soda from second effect condensate preheater530B flows through line L3B into second steam condensate preheater 540B,is heated further and exits through line L4B. The exit temperature ofthe aqueous caustic soda from the second steam condensate preheater 540Bis approximately 231° F.

The heat source for second steam condensate preheater 540B is thecondensed steam from first effect heating element 500B after beingfurther cooled by first steam condensate preheater 560B. The condensedsteam enters second steam condensate preheater 540B through condensateline C2B and exits through condensate line C3B. Condensate line C3Btransports the condensed steam back to and the boiler feed system to bereused as steam. The condensed steam entering second steam condensatepreheater 540B has a temperature of approximately 250° F. and an exittemperature of approximately 210° F. The flow rate of the steam isapproximately 36,500 lbs/hr.

Line 26B is connected to second product preheater 550B. The aqueouscaustic soda flowing through line L26B is heated in the second productpreheater 550B before exiting through line L5B at an approximatetemperature of 240° F. and an approximate flow rate of 183,300 lbs/hr.The heat source for second product preheater 550B is the final productaqueous caustic soda from first effect heating element 500B and firsteffect vapor body 505B after being cooled by first product preheater570B. The final product aqueous caustic soda enters second productpreheater 550B through line L10B at a temperature of approximately 260°F., is cooled and exits through line L11B at a temperature ofapproximately 158° F.

Line L5B and line L4B join and form line L30B. Line L30B joins with lineL19B, which is connected to second effect heating element 490B. Theaqueous caustic soda which is to be further concentrated enters secondeffect heating element 490B through line L19B. The aqueous caustic sodaflowing in line L30B has an approximate concentration of 40% causticsoda, an approximate temperature of 238° F., and an approximate flowrate of 233,300 lbs/hr. The aqueous caustic soda flowing through lineL18B has an approximate concentration of 44.2% caustic soda, anapproximate temperature of 226° F., and an approximate flow rate of844,700. The resulting aqueous caustic soda from the joining of linesL18B and L30B into line L19B has an approximate temperature of 229° F.,an approximate concentration of 43.3% caustic soda, and an approximateflow rate of 1,078,000 lbs/hr.

The aqueous caustic soda is heated in second effect heating element490B. Second effect heating element 490B is connected to second effectvapor body 495B. The heating of the aqueous caustic soda releases watervapor and results in remaining aqueous caustic soda having a higherconcentration of caustic soda.

The water vapor is removed from second effect vapor body 495B by vaporline V2B. Vapor line V2B joins with vapor line V5B from processcondensate receiver 90B and forms vapor line V6B. The water vaporremoved from second effect vapor body 495B flowing through vapor lineV2B and from process condensate receiver 90B flowing through vapor lineV5B combine in vapor line V6B and acts as the heat source for thirdeffect heating element 480B.

The concentrated aqueous caustic soda exits second effect heatingelement 490B and second effect vapor body through line L17B at anapproximate flow rate of 1,056,000 lbs/hr, an approximate temperature of226° F., and an approximate concentration of 44.2% caustic soda byweight.

Line L17B splits into lines L18B and L6B. Line L18B joins with line L30Bto create line L19B. Line L18B is part of a circulation line for secondeffect heating element 490B. The aqueous caustic soda in line L18B ispumped by pump 70B. The aqueous caustic soda in line L6B has atemperature of approximately 226° F., flow rate of approximately 211,300lbs/hr, concentration of approximately 44.2% caustic soda and proceedsto be further concentrated.

Line L6B splits into line L27B and line L28B. The aqueous caustic sodacontained in line L6B is pumped by pump 60B.

Line L27B is connected to first steam condensate preheater 560B. Theaqueous caustic soda which flows through line L27B at an approximateflow rate of 90,000 lbs/hr enters first steam condensate preheater 560B,is heated, and exits through line L7B with a temperature ofapproximately 296° F.

The heat source for first steam condensate preheater 560B is thecondensed steam from first effect heating element 500B which flowsthrough condensate line C1B at an approximate rate of 36,500 lbs/hr andan approximate temperature of 362° F. The condensed steam flows throughfirst steam condensate preheater 560B, is cooled and exits throughcondensate line C2B, which is connected to second steam condensatepreheater 540B.

Line L28B is connected to first product preheater 570B. The aqueouscaustic soda which flows through line L28B enters first productpreheater 570B and exits through line L8B at a flow rate ofapproximately 121,300 lbs/hr. The aqueous caustic soda is heated as itflows through first product preheater 570B from approximately 226° F. toapproximately 298° F.

The heat source for first product preheater 570B is the final productaqueous caustic soda from the first effect heating element 500B andfirst effect vapor body 505B. The final product aqueous caustic sodaenters first product preheater 570B through line L9B at an approximateflow rate of 186,700 lbs/hr, an approximate temperature of 320° F., andan approximate 50% concentration of caustic soda and exits through lineL10B at an approximate temperature of 260° F. As the final productaqueous caustic soda flows through first product preheater 570B, whereit is cooled.

Line L7B and line L8B join and form line L29B. The aqueous caustic sodain line L29B has an approximate temperature of 297° F. and anapproximate flow rate of 211,300 lbs/hr. Line L29B joins with line L15Bto form line L16B. The aqueous caustic soda flowing in line L15B has anapproximate concentration of 50% caustic soda, an approximatetemperature of 320° F., and an approximate flow rate of 131,100 lbs/hr,therefore, when joined with the aqueous caustic soda in line L29B, theresulting aqueous caustic soda in line L16B has an approximatetemperature of 306° F., an approximate flow rate of 342,400 lbs/hr, andan approximate concentration of 46.4% caustic soda by weight. Line L16Bis connected to first effect heating element 500B. The aqueous causticsoda flowing through line L16B flows into first effect heating element500B and is heated in order to release water vapor and concentrate theaqueous caustic soda further.

The heating source for first effect heating element 500B is steam whichenters first effect heating element through steam line S1B at atemperature of approximately 380° F., a pressure of approximately 194.7psia and a flow rate of approximately 36,500 lbs/hr. The steam afterbeing condensed as it proceeds through first effect heating element500B, exits through condensate line C1B. As discussed previously, thecondensed steam from first effect heating element 500B is further cooledas it flows through first steam condensate preheater 570B and secondsteam condensate preheater 540B before if flows back to the boiler tomake additional steam.

First effect heating element 500B is connected to first effect vaporbody 505B and it is from first effect vapor body 505B that the vapor isreleased. The vapor leaves first effect vapor body 505B through vaporline V1B at an approximate rate of 24,700 lbs/hr, an approximatepressure of 24.1 psia and an approximate temperature of 320° F. Thisvapor is the heat source for second effect heating element 490B.

The final product aqueous caustic soda exits first effect heatingelement 500B and first effect vapor body 505B through line L14B. Thefinal product aqueous soda in line L14B has an approximate concentrationof 50% caustic soda, an approximate flow rate of 317,800 lbs/hr, and anapproximate temperature of 320° F.

Line L14B splits into lines L15B and L9B. Line L15B connects with lineL29B. L15B is used to circulate a portion of the final product aqueouscaustic soda back to first effect heating element 500B. The finalproduct aqueous caustic soda which flows through line L15B is pumpedthrough pump 80B.

The final product aqueous caustic soda is pumped by pump 50B throughline L9B into first product preheater 570B, through line L10B, intosecond product preheater 550, and through line L11B. Through both firstproduct preheater 570B and second product preheater 550B, final productaqueous caustic soda is cooled.

Line L11B is connected to product cooler 100B. Final product causticsoda is cooled as it flows through product cooler 100B fromapproximately 158° F. to approximately 120° F. It exits product cooler100B through line L12B. Line L12B takes final product aqueous causticsoda to be stored, sold, or used in another process.

The cooling source for product cooler 100B is cooling water that flowsin through cooling water line CW5B and out through cooling water lineCW6B. The cooling water flows through product cooler 100B at a flow rateof approximately 1,000 gal/min and is heated from approximately 85° F.to approximately 96° F.

Cooling water is used to condense vapors from flash chamber 520B andfrom third effect vapor body 485B. Cooling water line CW2B is connectedto second surface condenser which condenses the vapors released fromflash chamber 520B. Cooling water flows through second surface condenser315B at an approximate flow rate of 8,000 gal/min and exits throughcooling water line CW3B at an approximate temperature of 96° F. Thecondensed vapor flows from second surface condenser 315B throughcondensate line C9B.

Condensate line C9B connects to process condensate receiver 90B and thecondensate vapor flows into process condensate receiver 90B at anapproximate temperature of 105° F. and an approximate flow rate of40,000 lb/hr.

Cooling water line CW1B is connected to surface condenser 305B to allowcooling water to enter surface condenser 305B and cool vapors releasedfrom third effect vapor body 485B. The condensed vapor leaves thesurface condenser 305B through condensate line C7B, which is alsoconnected to process condensate receiver 90B, at an approximatetemperature of 105° F. and an approximate flow rate of 18,400 lb/hr. Thecooling water flows through the surface condenser 305B at an approximaterate of 8,000 gal/min and is heated from approximately 85° F. toapproximately 90° F.

The cooling water leaves surface condenser 305B through cooling waterline CW4B.

Cooling Water lines CW3B, CW4B, and CW6B carry the cooling water backfor cooling such that it can be reused through cooling water lines CW1B,CW2B, and CW5B or in other parts of the facility.

Process condensate receiver 90B receives condensate from third effectheating element 480B though condensate line C6B, surface condenser 305through condensate line C7B, second surface condenser 315B throughcondensate line C9B, and second effect condensate preheater 530B throughcondensate line C5B. Vapors released in process condensate receiver 90B,if any, are removed through vapor line V5B, which joins vapor line V2Bas the heat source for third effect heating element 480B. The liquidcondensate, or water, is pumped from process condensate receiver 90B bypump 30B through condensate line C8B at an approximate temperature of131° F. and an approximate flow rate of 105,000 lb/hr. The water can beused within the plant or used as water source for the electrolytic cell.

The construction material of first effect heating element 500B, firsteffect vapor body 505B, first steam condensate preheater 560B, firstproduct preheater 570B, second effect heating element 490B, secondeffect vapor body 495B, second steam condensate preheater 540B, secondproduct preheater 550B and second effect condensate preheater 530Bshould be of a material that is resistant to corrosion by caustic soda,such as nickel. The construction material for the third effect heatingelement 480B and third effect vapor body 485B can be a higher grade ofstainless steel.

Flash chamber 520A and 520B can either be a flash chamber or anevaporator vapor body, both of which are known in the art.

Product cooler 100A and 100B, second effect condensate preheater 530Aand 530B, second product preheater 550A and 550B, second steamcondensate preheater 540A and 540B, first product preheater 570A and570B, first steam condensate preheater 560A and 560B, and catholyte heatexchanger 510A and 510B are heat exchangers known in the art forallowing heat to transfer between two liquids or a vapor and a liquid,depending on the composition of the materials flowing through theexchangers. Surface condenser 305A and 305B and second surface condenser315A and 315B are condensers known in the art for condensing vapors toliquids by using of cooling water.

Process condensate receiver 90A and 90B is a tank or chamber known inthe art for collecting condensate from several locations and bleedingoff any vapors that are released into the receiver due to the combiningof multiple condensate lines.

First effect heating element 500A and 500B, first effect vapor body 505Aand 505B, second effect heating element 490A and 490B, second effectvapor body 495B and 495A, third effect heating element 480A and 480Banti third effect vapor body 485A and 485B are evaporation systems knownin the art.

The above example gives an embodiment which attempts to optimize the useand recovery of heat from different heat sources as part of theconcentration process of aqueous caustic soda. However, those skilled inthe art will recognize that this invention can be practiced without theuse of second effect condensate preheater 530B, second stream condensatepreheater 540B, second product preheater 550B, first steam condensatepreheater 560B, first product preheater 570B, and product cooler 100B,or one of more of these devices can be incorporated.

Those skilled in the art will recognize that catholyte heat exchanger510B can consist of two or more heat exchangers arranged in eitherseries or parallel, and flash chamber 520B can consist of two or moreflash chambers connected to two or more vapor bodies arranged in eitherseries or parallel.

Further, those skilled in the art will recognize that the concentrationsof caustic soda flowing through this one embodiment can vary in actualpractice. For example, the concentration of the aqueous caustic sodaflowing through line L1B can range from approximately 31.0% toapproximately 33.0% caustic soda by weight. The concentration of theaqueous caustic soda flowing from flash chamber 520B and through lineL13B can range from approximately 36.0% to approximately 38.2% causticsoda by weight. The concentration of the aqueous caustic soda flowingfrom third effect heating element 480B and third effect vapor body 485Band through line L2B can range from approximately 38.8% to approximately41.2% caustic soda by weight. The concentration of the aqueous causticsoda flowing from second effect heating element 490B and second effectvapor body 495B and through line L6B can range from approximately 42.9%to approximately 45.5% caustic soda by weight. The concentration of thefinal product aqueous caustic soda flowing from first effect heatingelement 500B and first effect vapor body 505B and through line L9B canrange from approximately 48.5% to approximately 51.5% caustic soda byweight.

Although caustic soda is most commonly sold as a 50% concentrationproduct, either embodiment of the invention may be employed as aninitial step toward achieving caustic soda concentrations greater than50%, including the 70% and 100% commercial grades of caustic soda.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

1. A method of concentrating aqueous alkali comprising: providing anelectrolytic cell with a cation membrane; providing a catholytecomprising aqueous alkali with an initial alkali concentration from theelectrolytic cell; splitting the catholyte into a first stream and asecond stream wherein the first stream and the second stream have theinitial alkali concentration at a first temperature; providing anevaporation system including a set of effect evaporators and a flashevaporator; providing a catholyte heat recovery system; recovering heatfrom the first stream in the catholyte heat recovery system; flowing thesecond stream into the evaporation system; heating the second streamwith the catholyte heat recovery system; evaporating water from thesecond stream to create a third stream at a second temperature and afinal alkali concentration wherein the final alkali concentration isgreater than the initial alkali concentration; recovering heat from thethird stream to create a final product at a final temperature bycirculating the third stream through the catholyte heat recovery system;and adjusting the evaporation system to have the second temperaturegreater than the first temperature.
 2. The method of concentrating anaqueous alkali of claim 1 wherein: the step of evaporating water fromthe second stream includes concentrating the aqueous alkali in at leastone effect evaporator prior to recovering heat from the first stream. 3.The method of concentrating an aqueous alkali of claim 1 wherein: thestep of evaporating water from the second stream includes concentratingthe aqueous alkali in the flash evaporator; the step of recovering heatfrom the third stream is performed after concentrating the aqueousalkali in the flash evaporator; and, further concentrating the aqueousalkali in the set of effect evaporators.
 4. The method of concentratingan aqueous alkali of claim 1 wherein the step of providing anevaporation system includes providing a triple effect evaporator as theset of effect evaporators.
 5. The method of concentrating an aqueousalkali of claim 1 wherein the step of providing an evaporation systemincludes providing a double effect evaporator as the set of effectevaporators.
 6. The method of concentrating an aqueous alkali of claim 1wherein the step of providing an evaporation system includes providing asingle effect evaporator as the set of effect evaporators.
 7. The methodof concentrating an aqueous alkali of claim 1, further comprising:adjusting the evaporation system to have the first temperature at about190° F. and the second temperature at about 320° F.
 8. The method ofconcentrating an aqueous alkali of claim 1, further comprising adjustingthe flow rates of the first stream, second stream and third stream tocreate the final product stream at a flow rate of about 190,000 lb/hr.9. The method of concentrating an aqueous alkali of claim 1 furthercomprising: providing one or more heat exchangers, one or more flashevaporators and one or more evaporator bodies in the catholyte heatrecovery system.
 10. The method of concentrating an aqueous alkali ofclaim 1 further comprising: concentrating the aqueous alkali to betweenabout 70% and 100% alkali by weight.
 11. The method of concentrating anaqueous alkali of claim 1 further comprising: concentrating the aqueousalkali to about 50% alkali by weight.
 12. A system for concentrating anaqueous alkali comprising: an electrolytic cell with a cation membrane;a catholyte comprising the aqueous alkali with an initial alkaliconcentration in the electrolytic cell; a first effect evaporatorconnected by a first pipe system to the electrolytic cell; a flashevaporator connected by a second pipe system to the first effectevaporator; a plurality of effect evaporators connected by a third pipesystem to the flash evaporator; a heat recovery system connected to thethird pipe system; a first stream having the initial alkaliconcentration which flows from the electrolytic cell into the firsteffect evaporator; the first effect evaporator which evaporates waterfrom the first stream to create a second stream having a second alkaliconcentration; the second stream which flows from the first effectevaporator into the flash evaporator; the flash evaporator whichevaporates water from the second stream to create a third stream havinga third alkali concentration; the third stream which flows from theflash evaporator into the plurality of effect evaporators; the pluralityof effect evaporators which evaporates water from the third stream tocreate a fourth stream having a fourth alkali concentration; the fourthstream which flows from the plurality of effect evaporators through theheat recovery system; and, wherein the fourth stream exchanges heat withthe heat recovery system.
 13. The system of claim 12 further comprising:the plurality of effect evaporators comprising a second effectevaporator and a third effect evaporator connected by a fourth pipesystem; the third stream which flows from the flash evaporator into thesecond effect evaporator; the second effect evaporator which evaporateswater from the third stream to create a fifth stream; the fifth streamwhich flows from the second effect evaporator into the third effectevaporator; and, the third effect evaporator which evaporates water fromthe fifth stream to create the fourth stream.
 14. The system of claim 13further comprising: the heat recovery system which exchanges heatbetween the third stream and the fourth stream; and, the heat recoverysystem which exchanges heat between the fifth stream and the fourthstream.
 15. The system of claim 12 further comprising: the first streamwhich flows into the first effect evaporator at a first flow rate; and,a first recirculation system recirculating a sixth stream through thefirst effect evaporator at a second flow rate.
 16. The system of claim15 wherein the second flow rate is about seven times the first flowrate.
 17. The system of claim 12 further comprising: the second streamwhich flows into the flash evaporator at a first flow rate; and, asecond recirculating system recirculating a seventh stream through theflash evaporator at a second flow rate.
 18. The system of claim 17wherein the second flow rate is about seven times the first flow rate.19. The system of claim 17 further comprising: a heat exchangerconnected to the flash evaporator and to the electrolytic cell; aneighth stream having the initial alkali concentration circulatingthrough the electrolytic cell and the heat exchanger; and, wherein theseventh stream and the eighth stream exchange heat in the heatexchanger.
 20. The system of claim 19 wherein the eighth stream has aflow rate of about seven times the first flow rate.